In a great variety of abnormalities of the lower motor neuron it is of importance to determine the localization and the extent of the injury. It is also valuable to obtain a reliable and quantitative estimate of recovery of function or of the effect of treatment. A simple test has been devised which consists of application of a maximal electrical stimulus to the motor nerve and of recording, through the skin, the action potential of the muscle. An ordinary induction coil is suitable for stimulation, and the resulting electromyogram can be recorded on an electrocardiograph or any standard type of amplifier. Tests of this kind have been performed at an Army general hospital with which we are associated on patients with a large variety of peripheral nerve lesions during the past two years. In the present paper it is intended to describe the apparatus in detail and to point out